Chapter 17: Fire & Explosions

Fire

Type of oxidation which is combined with oxygen and other substances to produce new substances

Is a chain reaction

Oxidation reaction

Associated with the concept of energy

Energy

The ability of a system to do work

Takes many forms like heat & light

Exothermic Reactions

More energy is liberated than what is required to initiate the reaction

Ignition Temperature

The minimum temperature needed to ignite fuel

Heat of combustion

Heat involved when a substance burns

Combustion

Energy in form of heat and light (flame) is liberated, a portion of which is used to sustain fire

Initiate and sustain a fire

A fuel vapor, oxygen, heat

Fuel reaction rate

Oxygen sufficient to produce a flame only when it is in the gaseous state

The flash point

Lowest temperature at which a liquid produces enough vapor to burn

Wood burning

Only burns when exposed to heat hot enough to decompose into gaseous products (pyrolysis)

Glowing Combustion/Smoldering

Burning at the fuel-air interface such as: a cigarette, the embers of a wood fire, or a charcoal fire

Three mechanisms of heat transfer

Conduction, radiation, convection

Conduction

Movement of heat through a solid object

Poor conductors are called insulators

During a fire heat may transport through different metals creating a new fire at a different location

Radiation

Transfer of heat energy by electromagnetic radiation

Surface exposed to the heat of the fire may burst into flames when the surface reaches the ignition temperature

Convection

Transfer of heat energy by the movement of molecules within a liquid or gas

In a structural fire hot gases move to the upper portion of the structure causing surfaces to pyrolyze and burst into a fire

Flashover

Occurs when all the combustible fuels simultaneously ignite to engulf the entire structure

Arsons

Most arsons are started with petroleum based accelerants

Initiated by petroleum distillates such as gasoline and kerosene

Searching the fire scene

Must focus on finding the fire’s origin

Most productive in search for an accelerant or ignition device

Indicators of arson

Evidence of separate and unconnected fires

Use of “streamers” to spread fire

Irregular shaped pattern on the floor resulting from the pouring of accelerant onto the surface

Fires normally move in an upwards direction so it is most likely the origin of the fire will be the lowest point of the fire

Severe burning on the floor indicates flammable liquid

Combustible liquids are rarely consumed during a fire

Ignition devices

Most common is a match

Burning cigarettes

Firearms/ammunition

Mechanical match striker

Electrical sparking devices

Molotov cocktail

Collection of fire evidence

Ash, soot and porous materials should be stored in airtight containers such as paint cans or wide mouth glass jars

Never use plastic containers

Traces of flammable liquid residues may be located with a vapor detector (sniffer) or a trained canine

Substrate Control

Uncontaminated control specimens from another area of the fire scene

Recover flammable residues

Heat the airtight container in which the sample is in and then any volatile residue in the debris is driven off and trapped in the container’s enclosed airspace, the vapor is then removed with a syringe

When the vapor is injected into the gas chromatograph it is separated into its components and each peak is recorded on the chromatograph

Vapor Concentration Technique

Charcoal strip is placed in the airtight debris container when it is heated

The charcoal strip absorbs much of the vapors during heating

The strip is washed with a solvent which will recover the accelerant vapors

The solvent is then injected into the gas chromatograph for analysis

Gas Chromatography

Most sensitive and reliable instrument for detecting and characterizing flammable residues

Separates the hydro carbon components and produces a chromatographic pattern characteristic of a particular product

By comparing peaks recovered from the fire scene debris to known flammable liquids, it is possible to identify the accelerant used to initiate the fire

The chromatographic patterns by the unknown are compared to the patterns by petroleum products

IRLC

Ignitable Liquids Reference Hydrocarbon Collection

Shows the pattern of gasoline as well as many other accelerants

Gas Chromatography/Mass spectrometry

Complex chromatographic patters ca be simplified

Explosives

Substances that undergo a rapid oxidation reaction with the production of large quantities of gases

Sudden build up of gas pressure

Low Explosives

Black powder and smokeless powder

Black Powder

Mixture of potassium or sodium nitrate, charcoal, and sulfur

Smokeless powder

Nitrated cotton (nitrocellulose) or nitroglycerin and nitrocellulose

Confined to a container like a pipe

Deflagration

Speed of decomposition for low explosives

High Explosives

Primary explosives

Secondary Explosives

Dynamite, TNT, PETN, RDX

Primary Explosives

Ultra sensitive to heat, shock, or friction

Provide the major ingredients found in blasting caps or primers used to detonate explosives

Secondary Explosives

Relatively insensitive to heat, shock or friction

Burn rather than detonate if ignited in small quantities in the open air

Must be detonated by a primary explosive

Detonation

Speed of decomposition for high explosives

Produces super sonic shock wave creating a blast effect with an outward rush of gases at up to 7.000 miles per hour

Military and Peroxide explosives

Very popular for countries outside the US to us these for terrorist organizations

RDX

TATP

RDX

Most popular and powerful

Often encountered in the form of pliable plastic known as C-4

TATP

Homemade explosive used by terrorists

Combining acetone and peroxide in the presence of acid

Led to the banning of liquids on aircrafts

Triacetone triperoxide

Collection and Analysis (Explosives)

Must be searched thoroughly

Objects located at or near the origin of the explosion must be collected for examination

Loose soil and other debris must be preserved from its interior for analysis

Placed in airtight containers and labelled

Some explosives can diffuse through plastic and contaminate nearby containers

Crater

Located at the origin of the explosion

Ion Mobility Spectrometer

Screening objects for the presence of explosive residues

Back at the lab

Debris collected will be examined microscopically for unconsumed explosive particles

Debris may also be rinsed with organic solvents and analyzed by testing procedures that include colour spot test, thin layer chromatography, gas chromatography/mass spectrometry

Confirmatory identification tests may be performed on unexploded materials by an infrared spectrophotometer